The invention relates to a method and apparatus for measuring the mechanical properties, in particular yield strength, tensile strength and hardness, of metal cables or wires, in particular ferromagnetic cables or wires, during the production process or while in use, for lifting or conveying loads or supporting and transporting loads, for example in cable cars. The method to which the invention relates is also applicable for detecting aging in static applications, such as in pretensioned reinforced concrete or controlling the properties of tubular steel products such as pipes, and in all cases in which it is possible to produce relative motion between a control apparatus and the ferromagnetic structure that is being controlled.
It is of great importance, both for economical, technological and safety reasons, to be able to measure the mechanical properties, particularly the yield strength and tensile strength, of cables on line. While this is very useful in the production of the cables, to control their quality continuously and synchronously with the production rhythm, it is even more important when the cable is in use for lifting loads, such as in loading or unloading or salvaging operations, or for supporting and transporting persons, such as in cable cars, since the breakage of a cable may involve not only extreme economic loss, but also the loss of life.
The art provides no reliable method for measuring the yield strength or the tensile strength of steel cables. Destructive methods permit only testing of small portions of the cable and provide no protection against failures due to non-uniform properties of the cable. Further, they are not applicable to cables in use. Cables coming off a production line are tested using a small number of specimens and, once again, there is no way of accounting for non-uniformity between different specimens. Cables in use are tested only visually and it is obvious that visual testing is highly inadequate. The only alternative testing methods for steel cables, presently applied, are based on magnetic flux leakage and can only locate broken wires and loss of metallic area (LMA) of the cable. Therefore, the art does no permit control of the properties of an entire cable while in use.
A phenomenon known as the Barkhausen effect has been described extensively in the literature, e.g., in the paper by Swartzendruber et al. (J. Appl. Phys. 67, No. 9, 5469-5471 (1990). This effect consists of the large number of irreversible jumps in magnetization that occur while a ferromagnetic material is subjected to a varying magnetic field. The jumps are due to the unpinning of domain wall structures from defects and impurities in the material. As the applied field increases, a wall breaks away from one pinning site and moves rapidly until stopped by another pinning site. The resultant jumps in magnetization, which have the characteristic of a noise signal, can be detected by suitable sensors as described, for example, by Swartzendruber and Hicho in a separate paper (Res. Nondestr. Eval (1993) 5, 41-50).
Since the pinning sites of the domain walls, i.e., dislocations and impurities, also affect the mechanical properties of the steel, it is possible to correlate the Barkhausen signal with the mechanical properties of metals. Thus, the art describes the application of the Barkhausen effect to determine properties of metals based on relationships existing between the Barkhausen signals and said properties. For example, in U.S. Pat. Nos. 4,599,563, the Barkhausen signals are used to determine properties of steel. U.S. Pat. Nos. 4,689,558, 4,977,373 and 4,881,030 disclose the determination of residual stress or fatigue limit in steel and like materials, based on the Barkhausen signals. U.S. Pat. No. 5,313,405 discloses a system for non-destructive evaluation of the surface characteristics of a magnetic material, comprising probe means magnetically-coupled to a sample of the material for applying an alternating magnetic field at a plurality of frequencies selected to excite Barkhausen domains at different levels, near but below the surface of the sample, means for detecting a complex Barkhausen response of the sample for more than one frequency, and processor means for analyzing the characteristics of said responses to isolate at least frequency and amplitude information from each response for determination of the surface characteristics of the sample.
Other pertinent publications dealing with the measurements of mechanical properties of ferromagnetic materials are Karjalainen et al., "Detection of Fabrication Stresses by the Barkhausen Method, Effects of Fabrication Related Stresses", September 1985, pp. 149-161 and Kaplan et al., "Nondestructive Evaluation of Ferromagnetic Materials by Magnetometer-Like Experimental Arrangement", Journal of Nondestructive Evaluation, Vol. 6, No. 2, 1987, pp. 73-79.
In a recent paper by Thompson and Tanner (JMMM (1994)132, 71-88), measurements were made on low carbon steels of the Vickers hardness as a function of plastic deformation, and also of the coercivity as a function of plastic deformation, both to rather high values of deformation.
Coercivity can be measured by recording the magnetization of the sample as a function of variable applied magnetic field (hysteresis loop); in this case, the coercivity is determined as the value of applied field at which the magnetization of the measured sample is zero. The coercivity of ferromagnetic steel can also be estimated by the measurement of the Barkhausen signal rate (BSR) as a function of variable applied magnetic field. It can be shown that a maximam in the BSR occurs near the coercive field as the applied field is changed.
J. Kivimaa et al., "Influence of Tensile Stress in Steel Cables on Magnetic Barkhausen Noise, " IEEE Transactions on Magnetics, Vol. 29, no. 6, 1993, pp. 2992-2994, describes the application of the magnetic Barkhausen noise (BN) method for stress measurement in ferromagnetic steel cables. R. Ratitioaho et al., "Stress response of Barkhausen noise in high carbon steel cables and ropes", Journal of Magnetism and Magnetic Materials, vol. 1239, no. 2-3, pp. 217-225, describes a similar application in particular for steel concrete reinforcement cables. Both of these articles find that Barkhausen noise decreases with increasing tension. Neither of them, however, relates to the measurement of inherent mechanical properties of steel cables, particularly yield and/or tensile strength.
None of the prior art publications, however, disclose systems that are applicable to the on-line measurement of the mechanical properties, particularly yield strength and tensile strength, of elongated or filamentary structures, such as cables, of ferromagnetic materials, particularly steel. In particular, none of them discusses the effect of stresses and strains in the plastic range on the Barkhausen signal or any correlation between this signal and the mechanical propertes of steel cables. On-line control of steel cables, not only in the production line, but in actual use as well, is of paramount importance and presents difficulties which the prior art has not even considered. Spot checks and/or measurements providing average values of mechanical parameters provide no reliable information as to possible local defects or weak points, that may cause the failure of the cable when in use.
It should be noted that metal cables may fail due to one or more of the following causes:
abrasion and crushing, due to contact of the cable with an abrading medium or to its being subjected to severe mechanical pressure; corrosion, due to corrosive environment; broken or cut strands, due to fatigue or mechanical damage, or to a high overload beyond the load-bearing capacity of the cable; shock loading or overloading; and overheating or fatigue. PA1 creating at least a magnetic field that varies either as a function of time or as a function of position or both, along at least a given length of the structure; PA1 for each section of the structure to be tested, sensing a plurality of Barkhausen signals (hereinafter also indicated by "BS") due to the time or position related variation of said magnetic field; PA1 measuring the BS rate (hereinafter also indicated by "BSR") as a function of the value of the magnetic field applied to each said section; and PA1 determining the desired mechanical properties, particularly yield strength and tensile strength, of each such section from said sensed Barkhausen signals relative to it. PA1 means for generating a magnetic field that varies along at least a given length of the elongated structure; PA1 a plurality of Barkhausen signal sensors distributed along said structure length; PA1 means for comparing the signals relative to each section of the structure to be tested; and, optionally, PA1 computer means for elaborating the said signals to determine the relevant mechanical properties, e.g. to calculate yield strength and tensile strength, of each such section from said sensed Barkhausen signals relative to it.
Some of the above failure causes, e.g., overloading, overheating or fatigue, will affect the strength of the cable and still not be detected by current available non-destructive test methods.
It is therefore a purpose of this invention to provide a method and apparatus for determining the mechanical properties of cables of ferromagnetic material, that is applicable on-line, both in production and in use.
It is another purpose of this invention to provide such a method and apparatus which is applicable to a single wire as well as to a multiple strand cable.
It is a further purpose of this invention to provide such a method and apparatus which is applicable to multiple strand cables comprising strands of different ferromagnetic materials.
It is a still further purpose of this invention to provide such a method and apparatus that is effective in localizing weak sections of the cable where failure is likely to occur.
It is a still further purpose of this invention to provide such method and apparatus that is relatively inexpensive and simple to make.
It is a still further purpose of this invention to provide such a method which detects changes in the strength of the cable and will permit predicting the occurrence of events, which may lead to cable failure, even before they happen.
It is a still further purpose of this invention to provide such a method and apparatus that are effective in localizing sections of a cable where cable strength has changed or/and does not comply with the standard.
Other purposes and advantages of the invention will become apparent from the following detailed description.